Process for production of a silicon nitride ceramic

ABSTRACT

A silicon nitride ceramic of the present invention possesses excellent strength of the surface, including a silicon nitride and a rare earth oxide compound and being characterized in that the ratio of the transverse rupture strength, at a room temperature, of the fired surface used as a tensile surface to the transverse rupture strength, at a room temperature, of the worked surface used as a tensile surface subjected to the working so as to have the surface roughness of R MAX  0.8 μm or less is 0.7 or more, and the strength ratio is satisfied even when any portion besides the fired surface is utilized as the tensile surface to be worked to have the surface roughness of R MAX  0.8 μm or less. The present invention also provides a process for producing a silicon nitride ceramic including the steps of: (1) mixing α-Si 3  N 4  powder and β-Si 3  N 4  powder to obtain a raw material powder so as to satisfy the formula indicated by 0.05≦β/α+β≦0.50, in which a refers to the weight of α-Si 3  N 4  powder and β refers to the weight of β-Si 3  N 4  powder; (2) mixing at least one sintering aid to the raw material powder; (3) forming the powder mixture to give a compact; and (4) firing the compact at a temperature ranging from 1800° to 2000° C. under a nitrogen atmosphere having an atmospheric pressure of at least 1 atm.

This application is a divisional of application Ser. No. 08/589,171, filed Jan. 22, 1996, now U.S. Pat. No. 5,691,261, which is a continuation of application Ser. No. 08/437,216, filed May 8, 1995 (now abandoned), which is a continuation of application Ser. No. 08/179,685, filed Jan. 11, 1994 (now abandoned).

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a silicon nitride ceramic having high strength, and more particularly, to a silicon nitride ceramic having high strength on its surface without working after firing, and the process for production thereof.

A silicon nitride ceramic has conventionally been drawing attention because of its high strength at high temperatures, chemical stability, etc., and therefore, various researches have been made into the application of a silicon nitride ceramic to materials to be used for heat engines such as diesel engines and gas turbine engines.

Since silicon nitride generally has low sinterability, a rare earth oxide such as Y₂ O₃ is first added as a sintering aid to silicon nitride powder. Then, the powder is formed to make a compact having a desired shape. Finally, the compact is subjected to firing under gas pressure to obtain a silicon nitride ceramic.

As a raw material powder of silicon nitride to produce such a ceramic, there has been used a raw material containing 90% or more of α-Si₃ N₄ in consideration of the sinterability, etc. It is known that firing causes phase transition from α-Si₃ N₄ to β-Si₃ N₄ and forms columnar crystals.

However, firing the compact containing such a large amount of α-Si₃ N₄ brings about the roughness of crystallization and the like upon phase transition and does not give a minutely sintered ceramic, resulting in the deterioration of the strength and the like. From such a view point, Japanese Patent Laid-Open 62-297269, Japanese Patent Publication 2-50076, and Japanese Patent Laid-Open 2-255573 disclose a process for producing a silicon nitride ceramic, in which a large amount of β-Si₃ N₄ is added to a raw material powder.

A silicon nitride ceramic obtained by any of these processes (i.e., a process in which a raw material contains mainly α-Si₃ N₄ and a process in which a large amount of β-Si₃ N₄ is added to a raw material) possesses large pores on the surface of the ceramic just after firing (without working). (The surface without working after firing is hereinafter referred to as "fired surface".) The problem is that such a ceramic does not possess sufficient strength on the fired surface because stress concentrates in the peripheral portions of large pores.

Therefore, any of silicon nitride ceramics obtained by either of these two kinds of processes needs to be subjected to working onto the fired surface so as to produce a final products having the surface with desired strength. The step of working is laborious, time-consuming, and cost-increase.

Moreover, a ceramic obtained by any of the conventional processes does not possess satisfactory strength on the fired surface regardless of the thickness of the ceramic.

SUMMARY OF THE INVENTION

Hence, the present invention solves aforementioned problems. The present invention provides both a thin silicon nitride ceramic and a thick silicon nitride ceramic, each of which is excellent in the strength of the fired surface and each of which decreases the cost for producing the final product. The present invention also provides a process for producing such a silicon nitride ceramic.

The present inventors have earnestly studied, noting the properties of a silicon nitride ceramic such as the pore size and strength of the fired surface, and found that the above-described objects can be achieved by specifying the mixing ratio of β-Si₃ N₄ into the raw material powder, which led to the completion of the present invention.

Accordingly, the silicon nitride ceramic of the present invention, which substantially composed of a silicon nitride and a rare earth oxide compound, is characterized in that the ratio of the transverse rupture strength of the fired surface used as a tensile surface at a room temperature to the transverse rupture strength, at a room temperature of the worked surface used as a tensile surface subjected to the working so as to have the surface roughness of R_(MAX) 0.8 μm or less is 0.7 or more, and the strength ratio is satisfied even when any portion besides the fired surface is utilized as the tensile surface to be worked to have the surface roughness of R_(MAX) 0.8 μm or less.

R_(MAX) is a parameter for measuring surface roughness defined by JIS B 0601, which denotes the difference of the height between the highest point and the lowest point in a predetermined part of the curved line made by the cross section and the surface. Therefore, the relation between R_(MAX) and R_(a), which is another parameter for measuring surface roughness, can be expressed by the formula: R_(MAX) ≈4R_(a).

The process for producing a silicon nitride ceramic of the present invention is characterized in that α-Si₃ N₄ powder and β3-Si₃ N₄ powder are mixed to obtain a raw material powder so as to satisfy the following formula:

    0.05≦β/α+β≦0.50

(α refers to the weight of α-Si₃ N₄ powder, and β refers to the weight of β-Si₃ N₄ powder.) A sintering aid is added to and mixed with the raw material powder. Then, the powder is formed to give a compact. After that, the compact is fired at a temperature ranging from 1800° to 2000° C. under a nitrogen atmosphere having an atmospheric pressure of at least 1 atm.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is obtained a Si₃ N₄ ceramic, in which the difference of the surface strength between the fired surface and the surface subjected to a working (hereinafter referred to as a worked surface) is small, by controlling the compounding ratio of α-Si₃ N₄ powder and β-Si₃ N₄ powder.

Therefore, the obtained ceramic already possesses sufficient strength required to be used as a product before it is subjected to working unlike a silicon nitride ceramic obtained by any of conventional processes. When the compact is formed so as to have a near net shape which excludes a margin to be removed by working after firing, the obtained ceramic does not need to be subjected to working.

Thus, the strength on the fired surface is improved since a pore size in the ceramic becomes small by controlling the mixing ratio of β-Si₃ N₄ powder. Though the reason is not clear at present, it can be surmised as follows:

As the sintering mechanism of a Si₃ N₄ ceramic, first, SiO₂ particles and a sintering aid such as Y₂ O₃ particles, existing on the surface of Si₃ N₄ powder particles, melt and react to be in a liquid phase in the step of firing, and as the reaction proceeds, α-Si₃ N₄ particles and β-Si₃ N₄ particles, which were lying scattered at random before firing, are disposed regularly.

Then, all of the α-Si₃ N₄ particles and some of the β-Si₃ N₄ particles melt, and then precipitate and grow as columnar crystals of β-Si₃ N₄ around the remaining β-Si₃ N₄ particles (relatively large sized β-Si₃ N₄ particles) each of which works as a core for the crystal growth. The Si₃ N₄ ceramic seems to be obtained in this way.

In such a sintering mechanism, when a large amount of α-Si₃ N₄ powder is contained in the raw material powder, the amount of transitional α-Si₃ N₄ particles into β-Si₃ N₄ particles is large, and the amount of the remaining β-Si₃ N₄ particles, each of which works as a core for the crystal growth, is small. In this case, the strength of the fired surface of the obtained ceramic is lowered because the distribution of β-Si₃ N₄ crystals becomes uneven, which makes the size of pores rather large.

In contrast, when a large amount of β-Si₃ N₄ powder is contained in the raw material powder, the amount of transitional α-Si₃ N₄ particles into β-Si₃ N₄ particles is small, and the amount of the remaining β-Si₃ N₄ particles, each of which works as a core for the crystal growth, is large. In this case, the size of pores existing on the surface of the compact before firing does not become smaller as well as the diameters of the β-Si₃ N₄ particles after sintering become too small, and as a result, the pores become rather large, giving the same disadvantage.

Accordingly, it is presumed that the strength of the fired surface can be improved by controlling the amount of β-Si₃ N₄ powder contained in the raw material powder within the range given by the present invention because it makes the above mentioned transition from α-Si₃ N₄ to β-Si₃ N₄ appropriate and makes the pore size smaller.

Next, description is made on a silicon nitride ceramic of the present invention.

A silicon nitride ceramic of the present invention is characterized in that the difference between the strength of the fired surface and the strength of the worked surface is small. For example, when the surface of the ceramic of the present invention is worked with abrasive grains having a size of #800, the surface roughness indicated by Rmax becomes about 0.8 μm or less. The ratio of the four-point bending strength (transverse rupture strength) at a room temperature, of the fired surface used as the tensile surface to the four-point bending strength at a room temperature, of the worked surface used as the tensile surface is at least 0.7.

Further, this strength ratio is satisfied even when any portion besides the fired surface is utilized for the tensile surface to be worked. That is, even when a portion around the fired surface or around the center of the ceramic is utilized for the tensile surface to be worked, the above-described ratio is satisfied.

When the strength ratio is less than 0.7, the strength of the fired surface is not satisfactory, and it is not preferable to use such a fired surface as a surface of the final product from a practical view point. Further, the strength of the fired surface itself at a room temperature is preferably larger than 650 MPa, more preferably 700 MPa.

Furthermore, a pore size on the fired surface of the ceramic of the present invention is preferably not larger than 50×50 μm. "Pore size" is the size of pores existing in the cross section generated by a transverse rupture of the silicon nitride ceramic. When a pore is oval, the pore size is expressed by "a×b", referring to the length of the longer axis as a and the shorter axis as b.

When a pore exists on the tensile surface of the silicon nitride ceramic, the pore size is expressed by "a×b", referring to the length of the longer axis on the surface as a and the depth of the pore as b.

The significance of this measurement is based on the fact that such a rupture starts at such a pore when a ceramic in which the fired surface is utilized as the tensile surface.

A pore larger than 50×50 μm is not preferable because not only the strength but also the oxidation resistance of the ceramic deteriorate.

Further, R_(MAX) of the fired surface is preferably 15 μm or less. When R_(MAX) is larger than 15 μm, it is not preferable because the strength of the ceramic deteriorates.

Regarding the size of the columnar silicon nitride particles existing on the fired surface of the ceramic, it is preferable that the length perpendicular to the axis is 4 μm or less and the axis is 50 μm or less. When the size is larger than 4 μm×50 μm, it is not preferable because the strength of the ceramic is lowered since the columnar particles induce the rupture of the ceramic.

Moreover, concerning a sintered silicon nitride ceramic of the present invention, even a thick ceramic having a thickness of, for example 20-150 mm, as well as a thin ceramic, possesses excellent properties. Therefore, a silicon nitride ceramic of the present invention can be applied to various uses.

Next, the process for producing a silicon nitride ceramic of the present invention is described.

First, α-Si₃ N₄ powder and β-Si₃ N₄ powder are mixed so as to satisfy the following formula:

    0.05≦β/α+β≦0.50

(α refers to the weight of α-Si3N₄ powder, β refers to the weight of β-Si₃ N₄ powder.) It is preferable that the average diameter of the α-Si₃ N₄ powder particles is 0.3-1.0 μm and that the average diameter of the β-Si₃ N₄ powder particles is 0.3-3 μm. When the average diameter of the α-Si₃ N₄ powder particles is larger than 1.0 μm, the sinterability deteriorates. When the average diameter of the β-Si₃ N₄ powder particles is larger than 3 μm, the length perpendicular to the axis of the columnar crystals of β-Si₃ N₄ obtained by sintering becomes at least 3 μm, which is not preferable.

The amount of oxygen contained in α-Si₃ N₄ raw material powder can be increased by such a method that α-Si₃ N₄ raw material powder is fluidized in the reheating furnace the temperature of which is controlled to be, for example, 300° C.-800° C. for 1-20 hours and a method that α-Si₃ N₄ raw material powder is stirred in a slurry which has a temperature of 30°-80° C. and in which the amount of water is controlled.

The α-Si₃ N₄ is preferably treated by such a method because such a method further enhances the strength and the like of the Si₃ N₄ ceramic to be obtained.

To the obtained powder mixture is added 3-25 wt % of a sintering aid in proportion to the sum of the powder mixture and the sintering aid. The sintering aid is not particularly restricted, and, for example, a rare earth oxide such as Y₂ O₃, Yb₂ O₃, and Er₂ O₃, and a metal carbide such as MoC₂ and SiC, and the like can be employed. More than one sintering aid may be employed.

Then, the mixture is subjected to a mixed grinding using, for example, silicon nitride balls, followed by drying to obtain powder for forming a compact.

Then, the obtained powder is subjected to a molding such as press molding, extrusion molding, and slip casting to obtain a compact having a desired shape.

The obtained compact is then fired at a temperature of about 1800°-2000° C. under a nitrogen atmosphere having an atmospheric pressure of at least 1 atm for about 2-8 hours to obtain a sintered Si₃ N₄ ceramic.

The atmospheric pressure and the firing temperature is preferably controlled so that silicon nitride is not decomposed during firing. Further, the fired ceramic may be subjected to a heat treatment, if desired, at a temperature of about 1000°-1500° C. under an ambient atmosphere for about 0.5-10 hours.

The present invention is hereinafter described in more detail by way of Examples and Comparative Examples. However, the present invention is not restricted by the following Examples.

EXAMPLES 1-6

α-Si₃ N₄ powder having a purity of 97 wt %, an oxygen content of 1.40 wt %, and BET specific surface area of 12.0 m² /g and β-Si₃ N₄ powder having a purity of 98 wt %, an oxygen content of 0.8 wt %, and BET specific surface area of 9.0 m² /g are mixed in the ratio shown in Table 1.

To the mixture were added three kinds of sintering aids, Y₂ O₃ having an average particle diameter of 0.7 μm and BET specific surface area of 10.0 m² /g, MoC₂ having an average particle diameter of 1.5 μm and BET specific surface area of 2.0 m² /g, and SiC having an average particle diameter of 0.2 μm and BET specific surface area of 20.0 m^(2/) g.

The mixing ratio of each of the sintering aids, (i.e., Y₂ O₃, MoC₂, and SiC) is respectively 8 wt %, 2 wt %, and 1 wt % in proportion to the sum of the powder mixture and the sintering aids.

The obtained mixture of 20 Kg and silicon nitride balls of 40 g, which work as a medium for stirring, and water were put into a mill for stirring, having an internal volume of 100 liter, and the mixture was subjected to grinding for 3 hours. Subsequently, the mixture was dried to vaporize the water, and thus, granulated powder particles having an average diameter of 50 μm for forming a compact.

Then, the powder was subjected to press molding under the pressure of 200 kg/cm², followed by isostatic pressing under 5 ton/cm² to obtain a compact having dimensions of 80 mm×80 mm×6 mm (thickness). This compact was fired at a temperature of 1900° C. in a nitrogen atmosphere under an atmospheric pressure of 9.5 for 3 hours to obtain a silicon nitride ceramic having dimensions of about 67 mm×67 mm×5 mm (thickness).

(Performance Evaluation)

(1) Strength of Fired Surface

The fired surface of the silicon nitride ceramic was not subjected to working and used as a tensile surface. The ceramic was measured for four-point bending strength (transverse rupture strength) on the basis of JIS R 1601, except for tensile surface roughness, at a room temperature and at 1400° C. The results are shown in Table 1.

(2) Pore Size

The ruptured portions of the tesp piece which was measured for the strength of the fired surface were observed with a scanning electron microscope. At least samples were measured for the size of each pores existing on such portions. The average size of the examined pores are shown in Table 1.

(3) Size of Silicon Nitride Columnar Crystal

The fired surface of the silicon nitride ceramic was observed with a scanning electron microscope. 100 columnar crystals on the fired surface were chosen out at random. The average lengths parallel to the axis of the crystals and perpendicular to the axis of the crystals are shown in Table 1.

(4) Surface Roughness

The fired surface of the ceramic was measured for the surface roughness (R_(MAX)) using an instrument. The results are shown in Table 1.

(5) Strength of Worked Surface

The fired surface of the ceramic was subjected to working using grains having a size of #800 so that the surface roughness indicated by Rmax becomes 0.8 μm or less. The ceramic was measured for four-point bending strength at a room temperature and at 1400° C. At this time, the tensile surface was taken out of the vicinity of the center of the ceramic having a thickness of 5 mm. The results are shown in Table 1. Additionally, the ratio of the strength of the fired surface to the strength of the worked surface at a room temperature (σ_(as) /σ_(#800)) is also shown in Table 1.

Comparative Examples 1-7

The silicon nitride ceramics were obtained in the same manner as in Examples 1-6 except that the compounding ratio of α-Si₃ N₄ powder and β-Si₃ N₄ powder was varied as shown in Table 1. The ceramics were measured for the same performance evaluations as described above. The results are shown in Table 1.

EXAMPLES 7-14 Comparative Examples 8-11

The silicon nitride ceramics were obtained in the same manner as in Examples 1-6 except that the compounding ratio of α-Si₃ N₄ powder and β-Si₃ N₄ powder, the kind and the amount of sintering aids, and firing conditions were varied as shown in Table 2. The ceramics were measured for the same performance evaluation. The results are shown in Table 3.

The conditions of the sintering aids used were as follows:

Yb₂ O₃ Er₂ O₃ SC₂ O₃ WC NbC MoSi₂ WSi₂

Average Diameter(μm) 0.5 0.6 0.9 0.6 2.0 3.0 3.0

BET Specific

Surface Area(m² /g) 9.0 9.0 9.0 2.5 3.0 4.0 3.0

EXAMPLES 15, 16 and 20 Comparative Examples 12 and 13

To α-Si₃ N₄ powder was added water having 50 wt % in proportion to the sum of the weight of the powder and the water. The mixture was subjected to stirring treatment for 5 hours using an attriter mill so that the rate of the oxygen content in α-Si₃ N₄ powder is increased from 1.40 wt % to 1.50 wt %. The ceramics were obtained in the same manner as in Example 1-6 except for the kind and the amount of the sintering aids, which is shown in Table 4. The ceramics were measured for the same performance evaluations, and the results are shown in Table 5. The conditions of the sintering aids utilized are the same as the conditions described above.

EXAMPLES 17-19 Comparative Examples 14 and 15

The ceramics were obtained in the same manner as in Examples 1-6 except that the rate of the oxygen content in α-Si₃ N₄ powder was increased from 1.40 wt % to 1.55 wt % by the treatment at 500° C. in an electric furnace for 5 hours. The ceramics were measured for the same performance evaluations, and the results are shown in Table 4 and 5. The conditions of the sintering aids utilized are the same as the conditions described above.

EXAMPLES 21-24 Comparative Examples 16-18

The ceramics were obtained in the same manner as in Examples 1-6 except that there were utilized α-Si₃ N₄ powder having a purity of 97 wt %, an average particle diameter of 0.6 μm, an oxygen content of 1.3 wt %, and BET specific surface area of 12 m² /g (α-Si₃ N₄ content of 98%) and β-Si₃ N₄ powder having a purity of 98 wt %, an average particle diameter of 0.8 μm, an oxygen content of 1.0 wt %, and BET specific surface roughness of 9 m² /g (β-Si₃ N₄ content of 95%), and that the compounding ratio and the firing conditions were controlled as shown in Table 6.

The compacts having the dimensions of 80 mm×80 mm×24 mm (thickness), 100 mm×100 mm×60 mm (thickness), and 150 mm×150 mm×120 mm (thickness) were subjected to the treatment to obtain thick ceramics each having a thickness of 20 mm, 50 mm, and 100 mm respectively under the condition shown in Table 6. Each of the obtained ceramics were measured for the strength of the fired surface and the worked surface, and the results are shown in Table 7.

Regarding the measurement for the strength, two kinds of tensile surfaces were measured, i.e., (1) a worked surface obtained by abrasion to remove the thickness of 500 μm from the fired surface (outer portion of the ceramic), and (2) a portion just in the middle of the thickness of the ceramic (inner portion of the ceramic). The ratio of the strength on the fired surface to the strength of the tensile surface in outer portion (σ_(as) /σ_(out)) and inner portion (σ_(as) /σ_(in)) are also shown in Table 7.

                                      TABLE 1                                      __________________________________________________________________________                         Temp. of worked                                                                          Temp. of fired                                                                           Ratio of    Dimentions                         Compounding                                                                            Relative                                                                           surface (MPa)                                                                            surface (MPa)                                                                            strength at                                                                          Pore  of columnar                                                                          Surface                      ratio   density                                                                            Room      Room      room temp.                                                                           dimentions                                                                           crystal                                                                              roughness                    α *.sup.1                                                                    β *.sup.2                                                                     (%) temp.                                                                               1400° C.                                                                     temp.                                                                               1400° C.                                                                     σ.sub.ss /σ.sub.#800                                                     μm (μm)                                                                              R.sub.max            __________________________________________________________________________     Example                                                                              1 95  5   98.2                                                                               920  800  700  780  0.77  50 × 50                                                                        50 × 3                                                                         15                         2 90  10  99.3                                                                               930  800  840  790  0.90  50 × 40                                                                        30 × 3                                                                         8                          3 80  20  99.7                                                                               1000 830  800  800  0.80  15 × 15                                                                        35 × 4                                                                         11                         4 70  30  99.8                                                                               980  830  960  790  0.98  20 × 15                                                                        40 × 5                                                                         10                         5 60  40  99.3                                                                               950  790  810  780  0.85  30 × 20                                                                        30 × 3                                                                         9                          6 50  50  99.4                                                                               930  800  650  740  0.70  40 × 30                                                                        30 × 3                                                                         8                    Comparative                                                                          1 100 0   95.3                                                                               750  730  430  710  0.57  70 × 60                                                                        60 × 3                                                                         25                   Example                                                                              2 99  1   96.0                                                                               780  750  500  720  0.64  80 × 60                                                                        65 × 3                                                                         17                         3 40  60  99.5                                                                               900  740  570  700  0.63  60 × 70                                                                        20 × 4                                                                         6                          4 30  70  98.3                                                                               800  710  440  710  0.55  80 × 90                                                                        20 × 4                                                                         5                          5 20  80  99.7                                                                               760  710  430  650  0.57  70 × 60                                                                        10 × 3                                                                         7                          6 10  90  98.9                                                                               700  600  410  580  0.59   80 × 100                                                                      10 × 3                                                                         7                          7 0   100 99.4                                                                               650  550  300  600  0.46  105 × 60                                                                       10 × 3                                                                         7                    __________________________________________________________________________      *.sup.1 . . . α Si.sub.3 N.sub.4 powder                                  *.sup.2 . . . β Si.sub.3 N.sub.4 powder                             

                  TABLE 2                                                          ______________________________________                                         Com-                                                                           pounding     Amount of        Firing                                           ratio        sintering aid    conditions                                       α.sup.*1                                                                           β.sup.*2                                                                         (wt %)           °C. · hr ·          ______________________________________                                                                           atm                                          Example                                                                         7     95      5     12Y.sub.2 O.sub.3, 1Mo.sub.2 C                                                                1900 · 6 · 10             8     80     20     14Yb.sub.2 O.sub.2                                                                            1850 · 4 · 5              9     70     30     3Y.sub.2 O.sub.2, 13Yb.sub.2 O.sub.3,                                                         2000 · 3 · 10            10     90     10     10Er.sub.2 O.sub.2, 2Mo.sub.2 C, 2SiC                                                         1900 · 4 · 10                                                .sup.*3                                    11     50     50     5Sc.sub.3 O.sub.2, 2NbC                                                                       1900 · 2 · 10                                                .sup.*4                                    12     80     20     5Y.sub.2 O.sub.2, 5Er.sub.2 O.sub.3, 0.5WC,                                                   1950 · 4 · 10            13     70     30     5Y.sub.2 O.sub.2, 5Yb.sub.2 O.sub.2,                                                          1800 · 5 · 3             14     90     10     4Sc.sub.2 O.sub.2, 4Yb.sub.2 O.sub.3,                                                         1800 · 5 · 3             Compar-                                                                        ative                                                                          Example                                                                         8      0     100    6Y.sub.2 O.sub.2, 5SiC                                                                        1900 · 4 · 10             9     100     0     10Er.sub.2 O.sub.3, 2WSi2                                                                     1850 · 5 · 5             10     30     70     12Er.sub.2 O.sub.2, 6Yb.sub.2 O.sub.2, 3Mo.sub.2                                              2000 · 2 · 10                                                .sup.*5                                    11     10     90     3Y.sub.2 O.sub.2, 5Sc.sub.2 O.sub.3,                                                          1900 · 3 · 10            ______________________________________                                          .sup.*1 α Si.sub.3 N.sub.4 powder                                        .sup.*2 β Si.sub.3 N.sub.4 powder                                         .sup.*3 The test piece was worked and then subjected to heat treatment at      1300° C. for 5 hours.                                                   .sup.*4 The test piece was worked and then subjected to heat treatment at      1000° C. for 10 hours.                                                  .sup.*5 The test piece was worked and then subjected to heat treatment at      1500° C. for 1 hour.                                              

                                      TABLE 3                                      __________________________________________________________________________                 Temp. of worked                                                                         Temp. of fired                                                                          Ratio of    Dimentions                                   Relative                                                                           surface (MPa)                                                                           surface (MPa)                                                                           strength at                                                                          Pore  of columnar                                                                          Surface                                density                                                                            Room     Room     room temp.                                                                           dimentions                                                                           crystal                                                                              roughness                              (%) temp.                                                                              1400° C.                                                                     temp.                                                                              1400° C.                                                                     σ.sub.ss /σ.sub.#800                                                     μm (μm)                                                                              R.sub.max                      __________________________________________________________________________     Example                                                                              7 98.0                                                                               950 790  670 780  0.70  50 × 40                                                                        50 × 3                                                                         14                                   8 99.2                                                                               800 760  660 780  0.82  20 × 20                                                                        40 × 3                                                                         12                                   9 99.5                                                                               780 780  690 780  0.89  25 × 25                                                                        40 × 3                                                                         10                                   10                                                                               98.6                                                                               820 810  800 800  0.98  35 × 30                                                                        45 × 3                                                                         13                                   11                                                                               97.3                                                                               920 780  670 760  0.73  40 × 50                                                                        30 × 3                                                                         9                                    12                                                                               99.5                                                                               860 800  820 790  0.95  20 × 30                                                                        40 × 3                                                                         10                                   13                                                                               98.5                                                                               930 740  750 750  0.81  30 × 20                                                                        40 × 4                                                                         9                                    14                                                                               98.1                                                                               850 800  680 800  0.80  30 × 30                                                                        50 × 4                                                                         14                             Comparative                                                                          8 99.4                                                                               560 550  250 500  0.45  80 × 70                                                                        10 × 3                                                                         7                              Example                                                                              9 95.3                                                                               830 760  520 750  0.63  70 × 60                                                                        70 × 3                                                                         20                                   10                                                                               99.8                                                                               940 720  520 700  0.55  80 × 90                                                                        20 × 2                                                                         5                                    11                                                                               98.5                                                                               700 620  280 600  0.40   80 × 100                                                                      10 × 2                                                                         8                              __________________________________________________________________________      *.sup.1 . . . α Si.sub.3 N.sub.4 powder                                  *.sup.2 . . . β Si.sub.3 N.sub.4 powder                             

                  TABLE 4                                                          ______________________________________                                         Com-                                                                           pounding     Amount of        Firing                                           ratio        sintering aid    conditions                                       α.sup.*1                                                                           β.sup.*2                                                                         (wt %)           °C. · hr ·          ______________________________________                                                                           atm                                          Example                                                                        15     95     5      12Y.sub.2 O.sub.3, 1Mo.sub.2 C                                                                1900 · 6 · 10            16     80     20     14Yb.sub.2 O.sub.2                                                                            1850 · 4 · 5                                                 .sup.*3                                    17     90     10     4SO.sub.2 O.sub.2, 4Yb.sub.2 O.sub.3,                                                         1800 · 5 · 3             18     50     50     5Sc.sub.2 O.sub.2, 2NbC                                                                       1900 · 2 · 10                                                .sup.*4                                    19     70     30     5Y.sub.2 O.sub.3, 5Yb.sub.3 O.sub.3,                                                          1800 · 5 · 3             20     60     40     3Y.sub.2 O.sub.3, 14Yb.sub.2 O.sub.3                                                          2000 · 3 · 10            Compar-                                                                        ative                                                                          Example                                                                        12     70     30     8Y.sub.2 O.sub.3, 1SiC                                                                        1700 · 5 · 1             13     40     60     12Y.sub.2 O.sub.3, 1Mo.sub.2 C                                                                1900 · 6 · 10                                                .sup.*5                                    14     20     80     5Y.sub.2 O.sub.3, 5Yb.sub.2 O.sub.3,                                                          1800 · 5 · 3             15      0     100    10Y.sub.2 O.sub.3,                                                                            1900 · 4 · 10            ______________________________________                                          .sup.*1 α Si.sub.3 N.sub.4 powder                                        .sup.*2 β Si.sub.3 N.sub.4 powder                                         .sup.*3 The test piece was worked and then subjected to heat treatment at      1200° C. for 5 hours.                                                   .sup.*4 The test piece was worked and then subjected to heat treatment at      1300° C. for 5 hours.                                                   .sup.*5 The test piece was worked and then subjected to heat treatment at      1500° C. for 1 hour.                                              

                                      TABLE 5                                      __________________________________________________________________________                 Temp. of worked                                                                         Temp. of fired                                                                          Ratio of    Dimentions                                   Relative                                                                           surface (MPa)                                                                           surface (MPa)                                                                           strength at                                                                          Pore  of columnar                                                                          Surface                                density                                                                            Room     Room     room temp.                                                                           dimentions                                                                           crystal                                                                              roughness                              (%) temp.                                                                              1400° C.                                                                     temp.                                                                              1400° C.                                                                     σ.sub.ss /σ.sub.#800                                                     μm (μm)                                                                              R.sub.max                      __________________________________________________________________________     Example                                                                              15                                                                               99.0                                                                               1020                                                                               760  850 740  0.83  40 × 50                                                                        45 × 3                                                                         13                                   16                                                                               99.5                                                                               1100                                                                               810  990 800  0.90  30 × 10                                                                        50 × 3                                                                         12                                   17                                                                               98.5                                                                               1060                                                                               820  840 800  0.79  30 × 40                                                                        45 × 3                                                                         14                                   18                                                                               99.5                                                                               980 760  810 740  0.83  20 × 40                                                                        30 × 2                                                                         10                                   19                                                                               99.4                                                                               950 780  900 790  0.95  20 × 30                                                                        40 × 2                                                                         10                                   20                                                                               98.3                                                                               960 900  940 840  0.98  40 × 50                                                                        35 × 3                                                                         9                              Comparative                                                                          12                                                                               78.5                                                                               120 --   80  --   0.67  --    --    --                             Example                                                                              13                                                                               99.6                                                                               940 740  650 730  0.69  90 × 80                                                                        30 × 2                                                                         10                                   14                                                                               98.7                                                                               900 720  530 710  0.59  40 × 70                                                                        40 × 2                                                                         8                                    15                                                                               96.3                                                                               700 650  370 600  0.53  100 × 40                                                                       20 × 3                                                                         7                              __________________________________________________________________________      *.sup.1 . . . α Si.sub.3 N.sub.4 powder                                  *.sup.2 . . . β Si.sub.3 N.sub.4 powder                             

                                      TABLE 6                                      __________________________________________________________________________                                   Ceramic (5 mm in thickness)                                                        Temp. of worked Ratio of                     Compounding  Amount of Firing Relative                                                                           surface (MPa)                                                                           Temp. of fired                                                                        strength at                  ratio        sintering aid                                                                            Conditions                                                                            density                                                                            Room     surface (MPa)                                                                         room temp.                   α.sup.*1                                                                           β.sup.*2                                                                     (wt %)    °C. · hr · atm                                               (%) temp.                                                                              1400° C.                                                                     Room temp.                                                                            σ.sub.a s                                                                /σ.sub.#200            __________________________________________________________________________     Example                                                                        21    90  10 7Y.sub.2 O.sub.3, 1WC                                                                    1900 · 6 · 10                                                       99.0                                                                               900 780  720    0.80                         22    80  20 10Yb.sub.2 O.sub.3, 2Mo.sub.2 C                                                          1800 · 3 · 5                                                        99.5                                                                               860 750  730    0.85                         23    .sup.  70.sup.*3                                                                   30 5Y.sub.2 O.sub.3, 5Yb.sub.2 O.sub.3, 1SiC                                                1800 · 8 · 5                                                        97.8                                                                               1000                                                                               870  840    0.84                         24    60  40 4Y.sub.2 O.sub.3, 6Sc.sub.2 O.sub.3, 2SiC                                                1850 · 3 · 10                                                       99.0                                                                               920 830  740    0.80                         Comparative                                                                    Example                                                                        16    100  0 8Y.sub.2 O.sub.3, 1WC                                                                    1900 · 4 · 10                                                       96.8                                                                               800 760  440    0.55                         17    30  70 6Yb.sub.2 O.sub.3, 1Mo.sub.2 C                                                           1800 · 5 · 5                                                        99.3                                                                               800 710  470    0.59                         18    10  90 3Y.sub.2 O.sub.3, 8Sc.sub.2 O.sub.3, 2SiC                                                1850 · 3 · 10                                                       98.7                                                                               760 680  440    0.58                         __________________________________________________________________________      .sup.*1 α Si.sub.3 N.sub.4 powder                                        .sup.*2 β Si.sub.3 N.sub.4 powder                                         .sup.*3 α Si.sub.3 N.sub.4 powder was subjected to wet treatment to      increase the amount of oxygen from 1.3 wt % to 1.45 wt %.                

                                      TABLE 7                                      __________________________________________________________________________                  Outer portion of ceramic Inner portion of ceramic                                  Strength                                                                             Strength of        Strength                                                                             Strength of                                     of fired                                                                             worked surface                                                                          Ratio of  of fired                                                                             worked surface                                                                          Ratio of                           Relative                                                                           surface                                                                              (MPa)    strength at                                                                          Relative                                                                           surface                                                                              (MPa)    strength at                   Thickness                                                                           density                                                                            Room temp.                                                                           Room     room temp.                                                                           density                                                                            Room temp.                                                                           Room     room temp.                    (mm) (%) (MPa) temp.                                                                              1400° C.                                                                     σ.sub.as /σ.sub.#out                                                     (%) (MPa) temp.                                                                              1400° C.                                                                     σ.sub.as                                                                 /σ.sub.in       __________________________________________________________________________     Example                                                                              21                                                                               100  99.5                                                                               730   880 760  0.83  97.8                                                                               730   730 740  0.94                        22                                                                               50   99.6                                                                               740   880 770  0.84  98.9                                                                               740   860 780  0.86                        23                                                                               50   97.6                                                                               850   980 880  0.87  98.2                                                                               850   950 850  0.89                        24                                                                               20   99.5                                                                               750   860 750  0.87  99.5                                                                               750   880 700  0.85                  Comparative                                                                          16                                                                               100  98.3                                                                               430   810 770  0.53  90.5                                                                               430   590 350  0.73                  Example                                                                              17                                                                               50   98.5                                                                               430   820 730  0.52  95.3                                                                               430   630 600  0.68                        18                                                                               20   97.6                                                                               440   750 680  0.59  93.2                                                                               440   600 590  0.73                  __________________________________________________________________________

As shown in Tables 1-5, the silicon nitride ceramics in Examples 1-20, which are in the scope of the present invention, has a small difference in the strength between the worked surface and the fired surface and has small pore sizes. Moreover, the treatment to increase the rate of oxygen in α-Si₃ N₄ powder improves the properties of the ceramic to be obtained as shown in Table 5.

Further, a ceramic of the present invention maintains the above-described strength in both inner and outer portions, and shows excellent properties regarding the strength.

As have been described up to this point, the present invention provides a silicon nitride ceramic which is excellent in the strength of the fired surface and which decreases the cost for producing the final product by specifying the mixing ratio of β-Si₃ N₄ to the raw material. The present invention also provides a process for producing such a silicon nitride ceramic.

Therefore, the ceramic of the present invention does not need working after sintering so as to obtain satisfactory strength. When the ceramic is produced to have a near net shape, which does not have a margin to be removed by working, the ceramic does not need working.

Further, even a thick ceramic exhibits an excellent strength, and the application of the ceramic can be expanded. 

What is claimed is:
 1. A process for producing a silicon nitride ceramic, comprising the steps of:mixing a rare earth compound with α-Si₃ N₄ powder and β-Si₃ N₄ powder to obtain a raw material powder so as to satisfy the formula: 0.05≦β/α+β≦0.50 (α refers to the weight of α-Si₃ N₄ powder, β refers to the weight of -Si₃ N4 powder); forming said raw material powder to give a compact; and firing said compact at a temperature ranging from 1800° to 2000° C. under a nitrogen atmosphere having an atmospheric pressure of at least 1 atm to form a silicon nitride ceramic comprising silicon nitride and a rare earth compound, wherein the ratio of (1) the transverse rupture strength, at room temperature, of a fired surface used as a tensile surface to (2) the transverse rupture strength, at room temperature, of a worked surface used a tensile surface subjected to working so as to have a surface roughness of R_(MAX) 0.8 μm or less is 0.83 or more and wherein pores on the fired surface each have a size of 50 μm×50 μm or smaller.
 2. A process for producing a silicon nitride ceramic according to claim 1, wherein an oxygen content of α-Si₃ N₄ powder is increased within 1.40 to 1.70 wt %.
 3. A process for producing a silicon nitride ceramic according to claim 3, wherein said oxygen content is increased by fluidizing α-Si₃ N₄ powder in a reheating furnace the temperature of which is controlled to be 300° C.-800° C. for 1-20 hours.
 4. A process for producing a silicon nitride ceramic according to claim 2, wherein said oxygen content is increased by stirring α-Si₃ N₄ in a slurry which has a temperature of 30°-800° C. and in which the amount of water is controlled.
 5. A process for producing a silicon nitride ceramic according to claim 1, wherein said ceramic is subjected to a heat treatment at a temperature of 1000°-1500° C. under an ambient atmosphere for 0.5-10 hours.
 6. A process for producing a silicon nitride ceramic according to claim 2, wherein said ceramic is subjected to a heat treatment at a temperature of 1000°-1500° C. under an ambient atmosphere for 0.5-10 hours. 